1. Field of the Invention
This invention relates to a magnetoinductive flowmeter for moving fluids, incorporating a measuring tube, a magnet serving to generate a periodically alternating magnetic field that extends in a direction at least essentially perpendicular to the axis of the measuring tube, and two measuring electrodes positioned along a connecting line that extends in a direction at least essentially perpendicular to the axis of the measuring tube and to the vector of the magnetic field. The invention further relates to a magnetoinductive flow-measuring process for measuring the flow rate of a fluid moving through a measuring tube which is equipped with two measuring electrodes positioned along a connecting line that extends in an at least essentially perpendicular direction relative to the axis of the measuring tube, in which process a periodically alternating magnetic field is generated that extends in a direction at least essentially perpendicular to the axis of the measuring tube and perpendicular to the connecting line of the measuring electrodes, a voltage differentiated from a reference potential is collected at one or both of the measuring electrodes and the direct-current component of the voltage differentiated from the reference potential and collected at one or both of the measuring electrodes is quantified.
2. Description of the Prior Art
The basic principle of magnetoinductive flowmeters and of magnetoinductive flow-measuring techniques has been well known for some time and has been variously employed in a range of different applications. The underlying concept of a magnetoinductive flowmeter for moving fluids goes back to Faraday who in 1832 postulated the applicability of the principle of electrodynamic induction in the measurement of flow rates. According to Faraday""s law of induction, a moving fluid that contains charge carriers and flows through a magnetic field will develop an electric field intensity perpendicular to the direction of flow and perpendicular to the magnetic field. A magnetoinductive flowmeter utilizes Faraday""s law of induction by virtue of the fact that a magnet, usually consisting of two magnetic poles each with a field coil, generates a magnetic field perpendicular to the direction of the flow through the measuring tube. Within this magnetic field, each volume element of the fluid traveling through it and containing a certain number of charge carriers contributes its field intensity generated in that volume element to a measuring voltage that can be collected by way of measuring electrodes. In conventional magnetoinductive flowmeters, the measuring electrodes are so designed as to make conductive or capacitive contact with the moving fluid. A salient feature of magnetoinductive flowmeters is the proportionality between the measuring voltage and the flow rate of the fluid averaged across the diameter of the measuring tube, i.e. between measuring voltage and volumetric flow.
As stated above, a periodically alternating magnetic field is generated. In prior art, this has been accomplished following a variety of approaches. For example, a magnetoinductive flow measurement can be obtained using an alternating magnetic field for which purpose the field coils of the magnet are typically fed a sinusoidal 50/60 Hz line voltage directly off the power mains. However, transformation-induced noise and line interference compromise the measuring voltage derived from the flow between the measuring electrodes. Therefore, it is now common practice in magnetoinductive flow measurements to employ a polarity-reversible constant magnetic field. A polarity-reversible constant magnetic field is obtained by feeding to the field coils of the magnet a current with a sequential square-wave pattern which changes in periodically alternating fashion. Magnetoinductive flow measurements are also possible by means of a pulsed constant field which is obtainable by feeding to the field coils of the magnet a sequential square-wave current of unchanging polarity only at periodic intervals.
The magnetoinductive flowmeter and the magnetoinductive flow measuring technique referred to above have been described for instance in U.S. Pat. No. 5,677,496. According to that publication, the constant voltage component of the voltage collected on one or both of the measuring electrodes and differentiated from a reference potential is quantified for the purpose of subtracting its value from a flow-derived voltage that is composed of a value for the flow of the moving fluid through the measuring tube and a value for the said constant-voltage component. In this fashion, a voltage value is obtained which, now no longer error-prone due to the constant-voltage potential, provides a direct measure of the flow rate of the moving fluid through the measuring tube.
It is the objective of this invention to provide a magnetoinductive flowmeter and a magnetoinductive flow-measuring process, both as referred to above, which provide to the user specific information in addition to the flow rate.
Based on the concept of the magnetoinductive flowmeter described above, the magnetoinductive flowmeter according to this invention which achieves the stated objective is characterized in that it features an output device which displays a value representing the constant-current component of the voltage differentiated from the reference potential and collected at one or both of the measuring electrodes to the user, and/or makes that value available to the user for further processing.
The invention thus utilizes the fact that, apart from the magnetic-field-induced voltage, another voltage potential is generated at the measuring electrodes, that being the electrochemical noise voltage from which the user of the magnetoinductive flowmeter can derive additional, and additionally beneficial, information. These additional benefits, available to the user by virtue of this invention, are explained in detail further below in connection with the description of preferred embodiments of this invention.
The electrochemical noise voltage, utilized according to this invention, originates in contact potentials generated by the contact between the moving fluid and the measuring electrodes and resulting from the different chemical potentials of the moving fluid and the material of the measuring electrodes, respectively. Thus, according to the invention, the contact potentials which normally constitute a noise factor are utilized for providing to the user the possibility, in addition to flow-rate data, to derive quantitative information which is indicative of other parameters of the fluid, such as its conductivity or its charge-carrier concentration. In one example, a change in the electrochemical noise potential indicates a change of fluids in the measuring tube.
The above statement whereby an output device is provided for indicating a value for the constant-voltage component of the voltage collected, more specifically refers to an output which represents either the actual value of the constant-voltage component itself or a mean value for the constant-voltage component, or a value based on and derived from the constant-voltage component, such as the pH value of the fluid passing through the measuring tube.
In principle, each measuring electrode can have its own reference potential and the difference between the reference potential and the potentials of the individual measuring electrodes, the moving fluid, the measuring tube, etc. may be of any arbitrary magnitude. However, in a preferred embodiment of the invention, the reference potential employed is the earth potential. Especially when the earth potential is used as the reference potential, but in the case of other reference potentials as well, both measuring electrodes should preferably use the same reference potential. If, specifically, both measuring electrodes jointly use the earth potential as the reference potential, the structural design of the magnetoinductive flowmeter becomes quite uncomplicated and the cost associated with the evaluation of the measuring signals is correspondingly reduced.
It is entirely possible to use the voltage as differentiated from the reference potential and collected during one given cycle of the periodically alternating magnetic field for the direct quantification of the constant-voltage component. However, in a preferred embodiment of the invention, the value of the constant-voltage component differentiated from the reference potential and collected at a measuring electrode is summed and averaged over several cycles of the periodically alternating magnetic field. For unchanging flow conditions of the fluid, it will thus be possible to obtain an approximately constant value for the constant-voltage component since this approach significantly improves the signal-to-noise ratio.
Where the value of the constant-voltage component is to be determined via the two voltages collected at the measuring electrodes as differentiated from the respective reference potentials, the above-mentioned enhancement is obtained in a preferred embodiment of the invention by means of a summing device which, for the quantification of the value for the constant-voltage component in a measuring cycle, serves to determine the sum of the voltages differentiated from the reference potential and collected at the two measuring electrodes. In this connection, it is particularly desirable to additionally provide an averaging device by means of which, for the determination of the value for the constant-voltage component, the sums of the voltages differentiated from the reference potential and collected at the measuring electrodes can be averaged over several cycles of the periodically alternating magnetic field.
In a preferred design embodiment per this invention, defined conditions are obtained by additionally providing a grounding device for the moving fluid. Such a grounding device is preferably in the form of an earth electrode, earth rings and/or a pipe consisting at least in part of metal. While the same material can be used for the grounding device and for the measuring electrodes, a preferred embodiment according to this invention uses different materials for the grounding device and the measuring electrodes. This permits the utilization of effects which derive from the different contact potentials between the fluid passing through the measuring tube on the one hand, and the measuring electrodes and grounding devices made of different materials, on the other hand.
The output device of the magnetoinductive flowmeter may be in the form, for instance, of an optical display, or a bus system (field bus), or an output conductor and/or a frequency output port. Regardless of the type of output device, a preferred embodiment of this invention provides for any such output device to include a status indicator output with a threshold-value function. Thus, the output device does not, or not only, indicate a value for the constant-voltage component, but a status value from which it is evident whether or not a particular status exists. The status indicator is so designed as to capture and display a particular status as being present when the value measured exceeds or falls short of a predefined threshold value.
Another preferred embodiment of the invention incorporates a pH-calculating system by means of which, in the case of a known moving fluid and with the aid of the constant-voltage component of the voltage collected at one or both of the measuring electrodes and differentiated from the respective reference potential, the pH value of the fluid can be calculated. The pH-calculating system may be incorporated into the output device or it may equally well be a separate unit. Thus, in this preferred embodiment of the invention, the constant-voltage component of the voltage collected can be utilized for calculating the pH value, making it possible to perform an on-site calibration. The pH reading may be applied within the equipment and/or it can be retrieved at the output.
Finally, another preferred embodiment of the invention incorporates correction circuitry by means of which the value for the constant-voltage component of the voltage collected at one or both of the measuring electrodes and differentiated from the reference potential can be corrected as a function of the flow rate of the moving fluid. This takes into account that the constant-voltage component of the voltage collected depends on the flow rate of the fluid as it passes through the measuring tube. Errors otherwise introduced by this flow variable are thus largely avoided.
Based on the concept of the magnetoinductive flowmeter described above, the magnetoinductive flowmeter of this invention which achieves the stated objective is characterized in that the value determined for the constant-voltage component of the voltage collected at one or both of the measuring electrodes and differentiated from the reference potential is displayed to a user and/or made available to the user for further processing.
Preferred embodiments of the magnetoinductive flow-measuring process according to this invention are self-evident by analogous association with the above-described preferred embodiment of the magnetoinductive flowmeter according to this invention.
There are numerous specific ways in which the magnetoinductive flowmeter of this invention, and the magnetoinductive flow-measuring process according to the invention, can be designed and further enhanced. In this context, reference is made to the dependent claims and to the following detailed description of a preferred embodiment of the invention.